Non-emissive display technologies using electrochromic (EC) materials are of interest for devices with the ability to be viewed in a wide variety of lighting conditions, and for large area devices that can be fabricated using convenient inexpensive printing techniques. EC materials that can display three primary colors could be used to create full color display devices where the expression of any color can be achieved through the control of the intensity of each of the primary colors. Such devices require colors that can be switched “on” or “off” rapidly. Cathodically-coloring conjugated polymers are a class of materials that can demonstrate this rapid switching and polymers have been made that strongly absorb light in the visible region of the spectrum in a neutral state and upon oxidation absorb almost exclusively NIR radiation, leaving a near colorless, highly transmissive oxidized state. The addition of the colors red, green, and blue has been explored for displays from conjugated polymers; however, for truly non-emissive displays a primary subtractive color set, red, yellow and blue (RYB) or cyan, magenta and yellow (CMY), must be employed to produce all colors. Kobayashi et al., Solar Energy Materials and Solar Cells, 2008, 92, 136-9 has demonstrated a display from primary subtractive colored molecular electrochromic species, but the color system requires a very high potential (−2 V vs. Ag/AgCl) for switching and requires more than ten seconds and the sub-second switching speeds needed for device applications is not possible. Additionally, some colors are not stable to repeated switching.
The lack of yellow cathodically coloring conjugated polymers with band gaps of about 2.3 to about 2.8 eV has limited the development of multicolor display prototypes using primary subtractive colored conjugated polymers. The high band gap required to achieve a yellow color has prohibited switching from visible absorption bands in the neutral state to a transmissive NIR absorbing state upon oxidation. A few anodically coloring yellow conjugated polymers have been produced, Liou et al., Macromolecules, 2008, 42, 125-34 and Wang et al., Polymer Chemistry, 2010, 1, 1013-23. Although, in principle these conjugated polymers can be incorporated into display devices, in practice very specific potentials are required to achieve a yellow color without a brownish hue and these anodically coloring conjugated polymers have not displayed the potential for long term stability. The best electrochromic material that displays a yellow color consists of a conjugated oligomer containing pendant acrylate groups that can be synthesized and patterned by a UV mediated process, Nielsen et al., Journal of the American Chemical Society, 2008, 130, 9734-46. The conjugated oligomer segments of the polyacrylate switch from a yellow neutral state to an intense blue cation radical state and ultimately a less intense but still strongly blue dicationic state.
A yellow to transmissive cathodically coloring conjugated polymer is very desirable as it is the only color missing from the primary subtractive colored conjugated polymers. A non-emissive conjugated polymer display having a primary subtractive color set can be produced with a yellow to transmissive cathodically coloring conjugated polymer.